Insights into the Structural Transformation of Amorphous ZrP, α-ZrP, and γ-ZrP: Sr²⁺ Sorption Behavior and Mechanism

This study investigates the synthesis and Sr²⁺ sorption properties of three different crystal types of zirconium phosphate (ZrP)─amorphous ZrP (am-ZrP), α-ZrP, and γ-ZrP─prepared via a one-pot reflux method. The materials were characterized by using X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared (FTIR) spectroscopy, thermogravimetry (TG), solid-state ³¹P magic angle spinning nuclear magnetic resonance (NMR) spectroscopy, and sodium hydroxide titration. Sorption experiments revealed that Sr²⁺ uptake by all three ZrPs increased with pH. Sorption isotherms were evaluated across a range of Sr²⁺ concentrations, with maximum sorption capacities at pH 9 of 2.48 mmol g^-1 for am-ZrP, 2.71 mmol g^-1 for α-ZrP, and 2.04 mmol g^-1 for γ-ZrP. Sorption kinetics and the influence of competing ions on Sr²⁺ uptake were also examined. Notably, all three ZrP materials demonstrated excellent reusability in sorption-desorption cycles. Finally, the structural transformation and Sr²⁺ sorption mechanism were elucidated through ³¹P NMR, XPS, FTIR and TEM analyses.